What is to be said about the declension has already been said here: 3.2.13 The combinaison ck. Open the video and look at the grammar explanations. The information you find there is an introduction to the topic, the details are a little bit more complicated and will be reconsidered in chapter 9), the declension in german, chapter 10), the pronouns, chapter 13), some prepositions, chapter 16), possessive pronouns etc. etc. The declension plays an important role in German. For the moment we just need to know what it is and have a general idea how it works.

Nach dem ganzen Obst jetzt mal was ganz anderes.
After all that fruit now something different.

Hier hab ich ein Blatt.
Here I have a leaf.

Nominativ
Das ist das Blatt.
Wer oder was?
Das ist das Blatt.

nominative
This is a leaf.
What is that?
This is a leaf.

Genitiv
Ich mag die Farbe des Blattes.
Wessen Farbe?
Ich mag die Farbe des Blattes.

genitive
I like the colour of the leaf.
Whose colour do I like?
I like the colour of the leaf.

Dativ
Was ich hier erzähle, ist dem Blatt egal.
Wem ist es egal?
Was ich hier erzähle, ist dem Blatt egal.

dative (in English it is not a dative)
The leaf doesn' t care what I say.
Who doesn' t care what I say?
The leaf doesn' t care what I say.

Akkusativ
Ich sehe mir das Blatt an.
Wen oder was?
Ich sehe mir das Blatt an.

acusative
I look at the leaf.
What do I look at?
I look at the leaf.